A trend in image processing and computational photography is to obtain additional optical information at the time images are acquired. This enables greater post-acquisition image processing applications, such as deblurring, refocusing, and novel images.
The key idea is to acquire the entire 4D light field entering the camera, via the lens, and incident on the camera sensor. The light field is a function that describes the amount of light traveling in every direction through every point in space, Gershun, “The Light Field,” Translated by Moon et al., Journal of Mathematics and Physics, Vol. XVIII, MIT, 1939, pp. 51-151, 1939. In a conventional camera, the sensed 2D image is only a 2D projection of the 4D light field. Therefore, it is not possible to reconstruct the entire 4D light field. Therefore, the way that conventional images can be changed in a meaningful way is severely limited.
Using a complex arrangement of optical elements, e.g., multiple lenses, mirrors, beam splitters and/or sensors, it is possible to re-bin the 4D rays and acquire them using a 2D sensor, Georgiev et al., “Spatio-angular resolution trade-offs in integral photography,” EGSR, pp. 263-272, 2006. There a lens array with multiple optical paths performs the optical implementation of the two plane parameterization of the light field, see Levoy et al., “Light field rendering,” SIGGRAPH 96, pp. 31-42, 1996, and Gortler et al., “The lumigraph,” SIGGRAPH, pp. 43-54, 1996. However, optical rebining of rays forces a fixed and permanent tradeoff between spatial and angular resolution via the array of lenses.
Light Field Acquisition: Integral Photography
Instead of measuring each incoming direction separately to estimate the entire 4D light field function, light field acquisition was first described about a century ago to “undo” the directional integration of all rays arriving at one point on a 4D film or sensor plane. A survey of the first integral cameras and its variants is described by Okano et al., “Three dimensional video system based on integral photography. Optical Engineering 38, pp. 1072-1077, 1999.
The concept of the 4D light field as a representation of all rays of light in free-space was described by Levoy et al., and Gortler et al. While both created images from virtual viewpoints, Levoy et al., also described computing images through a virtual aperture. However, a practical method for computing such images was not demonstrated until after a thorough study of 4D interpolation and filtering by Isaksen et al., “Dynamically reparameterized light fields,” SIGGRAPH, pp. 297-306, 2000. Similar methods have also been called synthetic aperture photography, Vaish et al., “Using plane+parallax for calibrating dense camera arrays,” Proc. Conf. Computer Vision and Pattern Recognition, pp. 2-9, 2004.
To acquire the 4D light field by a 2D sensor, the following two techniques are most often used. The first technique uses an array of lenses to acquire the light field of the scene from a grid of viewpoints. The image formed behind each lens provides an orderly grid of angular samples to provide a result similar to integral photography. Instead of fixed lens arrays, an optically equivalent configuration of individual cameras can also be used. There, an array of positive lenses is placed in front of a conventional camera with the aid of a prism.
The second technique places a single large lens in front of an array of micro-lenses and treating each micro-lens for spatial samples. Those plenoptic cameras form an image on the array of micro-lenses, each of which generates an image that samples an angular distribution of radiance at that point. This technique interchanges the placement of spatial and angular samples on the image plane.
Both of the above techniques trade spatial resolution for the ability to resolve angular differences. They require very precise alignment of micro-lenses and optical path with respect to the sensor. Obviously, those techniques are not suited for a simple conventional (digital) cameras, with just a lens, aperture and sensor.
Coded Imaging
In astronomy, coded aperture imaging is used to overcome the limitations of a pinhole camera, Skinner, “X-Ray Imaging with Coded Masks,” Scientific American 259 p. 84, August 1988. Modified Uniformly Redundant Arrays (MURA) have been used to code the light distribution of distant stars. However, aperture imaging precludes the use of lenses as are used in conventional cameras.
A coded exposure camera can preserve high spatial frequencies in a motion-blurred image and make the deblurring process well-posed, see Raskar et al., “Coded exposure photography: motion debt lining using fluttered shutter,” ACM Trans. Graph. 25, 3, pp. 795-804, 2006, and U.S. patent application Ser. No. 11/430,233, “Method and Apparatus for Deblurring Images” filed by Raskar on May 8, 2006, both incorporated herein by reference. That technique uses temporal modulation to minimize motion blur in images.
Prior art optical systems involving lenses and coded masks are rather limited. One system places a mask with four pin holes in front of the main lens and estimate depth from defeats by capturing four images, Hiura et al., “Depth, measurement by the multi-focus camera,” CVPR '98: Proceedings of the IEEE Computer Society Conference on Computer Vision and Pattern Recognition, IEEE Computer Society, Washington, D.C., USA, p. 953, 1998. However, it is desired to only use a single image for light field acquisition.
Wavefront coding is another technique to achieve extended depth of field (DOF) that use aspheric lenses to produce images with a depth-independent blur, Dowski et al., “Wavefront coding: A modern method of achieving high performance and/or low cost imaging systems,” SPIE Annual Meeting, 1999. While their results extend depth of field in images, their design cannot provide a light field.